@arichardson pointed out in post-commit review for
https://reviews.llvm.org/D95583 (b714f73defc8e075) that `-verify` has an
optional argument that works a lot like `FileCheck`'s `-check-prefix`.
Use it to simplify the test for `-fno-implicit-modules-use-lock`!
Add -cc1 flags `-fmodules-uses-lock` and `-fno-modules-uses-lock` to
allow the lock manager to be turned off when building implicit modules.
Add `-Rmodule-lock` so that we can see when it's being used.
Differential Revision: https://reviews.llvm.org/D95583